Foamed polypropylene sheet having improved appearance and a foamable polypropylene composition therefor

ABSTRACT

Foamable polypropylene composition comprising a polypropylene resin having an D1238L melt flow rate of from about 0.5 to about 30 g/10 min, and a method for extruding rigid, foamed polypropylene sheet with improved surface appearance having a density in the range 0.4 to about 0.8 g/cm3

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 09/387,330 filed Aug. 31,1999.

This application claims the benefit of U.S. Provisional Application No.60/098,958, filed Sep. 3, 1998, U.S. Provisional Application No.60/122,129, filed Mar. 1, 1999, and U.S. Provisional Application No.60/128,173, filed Apr. 6, 1999.

BACKGROUND OF THE INVENTION

This invention relates to polyolefins, and more particularly to improvedexpanded or foamed compositions comprising propylene polymers. Stillmore particularly, the invention relates to rigid or semi-rigidpolypropylene foam sheet having improved surface appearance.Polypropylene foam sheet according to the invention is readilythermoformable into shaped articles that are particularly useful inrigid and semi-rigid packaging and in fabricating trays, plates,containers and other articles used in food service applications.

Polystyrene has found wide acceptance for use in food serviceapplications because of its good rigidity and shape retention and, asfoam sheet, it is readily molded and thermoformed. However, polystyrenearticles suffer from low service temperature, and generally are fragileand lack chemical resistance. The food service and packaging arts havelong sought alternative materials that do not have these undesirablecharacteristics.

Polyolefin resins are widely known for their ease of fabrication and arefound in a great variety of applications. Considerable effort has beenexpended in recent years to develop rigid expanded or foamed polyolefinsheet as a replacement for styrene foams, particularly for use in foodservice applications. Polyethylene resins have moderate strength andhigh toughness, with softening temperatures in the range of from 105° to140° C. Such resins are suitable for foam extrusion, giving attractive,low density, thermoformable foam sheet with good low temperatureproperties. Foam polyethylene sheet and molded foam articles withexcellent surface appearance are readily produced from low densitypolyethylene resins and so have found wide acceptability for use in avariety of packaging applications as well as in food serviceapplications. However, polyethylene foams generally are soft andflexible and have poor heat resistance, and thus may find limitedacceptance for food service uses requiring rigidity, and where contactwith hot foods is contemplated.

Propylene polymers, or polypropylene resins, are particularly noted fortheir good heat resistance and mechanical properties, and resinformulations based on polypropylene are supplied to meet the demandsimposed by a variety of structural and decorative uses in the productionof molded parts for appliances, household goods and autos. Impactmodified polypropylene and elastomeric ethylene-propylene copolymershave found application in automotive applications including interiortrim as well as in exterior parts such as bumper facia, grillcomponents, rocker panels and the like. Polypropylene resins have thethermal and chemical resistance to withstand exposure to the widevariety of environments and are easily molded at a cost far below thatof metal stamping to provide parts that will not rust or corrode and areimpact resistant, even at low temperature.

A number of processes for producing polypropylene foam have beendisclosed and are well described in the art, including for example themethods disclosed in U.S. Pat. No. 5,180,571 to J. J. Park et al. andthose set forth in the references cited and summarized therein. The Parket al. patent is directed to the extrusion of polypropylene to providefoam sheet having a low density generally in the range of from 0.04 toabout 0.44 g/cm³.

At the surface of extruded foamed polypropylene sheet there generallymay be found a layer consisting substantially of crystallinepolypropylene (PP). This surface layer or skin is important to partappearance and surface hardness. The thickness and crystallinity of thePP surface layer that forms depends in part upon extrusion conditionsincluding die temperature and cooling rates, and upon annealing. ThePark et al. patent is directed to the extrusion of polypropylene toprovide foam sheet with a smooth surface skin and a uniform cellstructure. According to the teachings of Park et al. it is necessary touse high melt strength, high melt elasticity polypropylene with aparticularized combination of molecular and rheological characteristicsincluding bimodal molecular weight distribution and a minor componentthat is highly branched to produce foam sheet having a smooth surfaceskin and a uniform cell structure. Patentees provide comparisons showingthat low density foam sheet extruded using conventional or genericpolypropylene resins, further characterized as polypropylene resins withmonomodal molecular weight distributions and an absence of significantbranching, generally have roughened sheet surfaces and non-uniformmicrocellular structure and are unacceptable for commercial use.

The surfaces of extruded polyolefin foam sheet generally lack thesmooth, shiny, uniform and substantially unblemished surfaces observedwith extruded styrenic foam sheet, particularly including higher densityABS foam sheet. For example, surface roughness is commonly encounteredwhen extruding polyethylene foam sheet, and lack of uniformity in cellstructure and distribution at the surface is visually more readilyapparent because of the transparent nature of unfilled polyethylene.Sensible surface roughness, that is, roughness that can be sensedtactilely, may be reduced by contacting the lower melt temperaturepolyethylene sheet with a polishing roll during the extrusion process togive a smooth, more even surface. The surface imperfections that remainare mainly visible density variations and are generally uniformlydistributed, providing a textured or marbleized surface appearance thatis pleasing and generally acceptable.

Rigid polypropylene foam sheet obtainable by the processes currentlyknown and practiced in the art continues to be somewhat lacking insurface appearance characteristics. Characteristically, polypropylenefoam sheet extruded with conventional processes and using conventionalor generic polypropylene resins will have regularly spaced markings inthe form of alternating bands or corrugation-like markings extending thelength of the sheet in the machine direction. In light, low-densityfoams obtained from conventional polypropylene resins, particularlysoft, flexible foams having a densities of 20 lb/ft³ (0.3 g/cm³) andlower, these bands may have the form of a regularly spaced, wave-like orsinusoidal distortion, forming a corrugated sheet. The bands orcorrugations become less pronounced for rigid foam sheet and,particularly at higher foam densities, are seen as surface flaws orappearance defects that take the form of linear, valley-like surfacedepressions along the machine direction.

The surface roughness of sheet extruded using these higher meltingresins is more difficult to smooth adequately using a polishing roll.Moreover, the imperfections and visible density variations found in thesurfaces of extruded polypropylene foam sheet are often not uniformlydistributed over the surface, and are generally quite visible, even forsheet that otherwise is tactilely smooth. In rigid, higher density foamssuch as are desirable for the production of food service articles thedefects more often appear as a pattern of alternating linear bands ofhigh and low foam density, characterized by readily visible variationsin translucence and surface gloss, possibly including surface voids,bubbles, streaks and uneven color. Such flaws may be without significanteffect on the mechanical properties of the foam, and generally do notaffect the performance of finished goods fabricated from such foam.However, in consumer goods, food packaging and the like, these visiblesurface defects and related cosmetic flaws are highly undesirable, thuslimiting acceptance of rigid polypropylene foam sheet by the industry.

Coextrusion of multilayer sheets having solid outer skins and a foamedcore has been disclosed in the art and is widely used to overcomesurface appearance problems encountered in the production of a varietyof prior art foam sheet materials including those made from polystyreneand ABS. Foam core sheet, provided with a shiny or glossy unfoamedsurface layer formed of the same or another resin, may be improved inresistance to surface abrasion and cuts and have a superior appearance.The more rigid skin serves to stiffen the foam structure, allowing alighter and thinner structure while attaining maximum bending stiffness.Foam sheet coextrusion processes are well described in the art for usewith a variety of resins such as polystyrene and ABS, and methods havebeen recently disclosed for use in the coextrusion of multilayered foamsheet comprising polyolefins including polypropylene. Coextrusionprocesses suffer the disadvantage of generally requiring more costlyfeedblocks, dies and related machinery having a more complicated design,thereby increasing the complexity of the operation and raising the costof producing such foam sheet.

Thus, there continues to be a need for rigid, foamed polypropylene sheetcomprising conventional generic polypropylene resins with reducedvisible surface defects and related cosmetic flaws and having theattractive, defect-free surface appearance necessary for acceptance inthe food service and packaging industries, and for polypropylenecompositions comprising conventional generic polypropylene resins thatare suitable for use in the extrusion of such foam sheet.

SUMMARY OF THE INVENTION

The invention pertains to the production of rigid, foamed polypropylenesheet significantly improved in surface appearance. More particularly,the invention relates to improved foamable polypropylene compositionscomprising a propylene polymer, a crystallization nucleating agent, abubble nucleating agent, and a blowing agent for use in the productionof rigid, foamed polypropylene sheet having a high density, greater thanabout 0.45 g/cm³, with excellent strength and thermal insulatingproperties. When extruded employing the improved extrusion apparatus anddie according to the further teachings' set forth herein, the resultinghigh density, foamed polypropylene sheet may be characterized as havinga more uniform cellular structure together with an improved surfaceappearance including significant reduction in the surface banding,corrugation and related visible flaws commonly encountered in extruded,high density, rigid foam sheet; the invention thus may be furthercharacterized as directed to improved extruded, rigid, foamedpolypropylene sheet.

Rigid, foamed polypropylene sheet according to the invention has a lowaverage surface roughness uniformly distributed over the surface of thefoam sheet, and a substantial absence of corrugation and surfacebanding. When molded or otherwise thermoformed, the invented foam sheetwill afford rigid or semi-rigid articles having improved surfaceappearance while retaining a good balance of mechanical propertiesincluding stiffness and toughness. The invention thus may also becharacterized as directed to molded articles having improved appearancecomprising expanded or foamed polypropylene.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammatic, perspective view, partially in phantom, of atypical coathanger-type sheet extrusion die.

FIG. 2 is a sectional view of a prior art polyolefin sheet extrusiondie, taken along line 2—2 of FIG. 1.

FIG. 3 is a sectional view, taken along line 2—2 of FIG. 1, showing animproved sheet extrusion die according to the invention.

FIG. 4 is a sectional view, taken along line 2—2 of FIG. 1, showing analternative embodiment of the improved sheet extrusion die according tothe invention.

FIG. 5 is a fragmentary, enlarged sectional view showing the detail ofthe die land portion of the embodiment of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The particular advantage of the invention as disclosed herein is thathigh density foam sheet having excellent surface appearance may beprovided using any of the great variety of commercially availablepolypropylene resins, and without resorting to specialty resin gradesand blends. The polyolefin compositions useful in the extrusion ofthermoformable, rigid, foamed polypropylene sheet according to theinvention will preferably comprise a substantially linear polypropylenehomopolymer, or a copolymer of propylene and a minor amount, up to about30 wt. %, more preferably up to about 20 wt. % of an alpha-olefin whichmay have up to 6 carbon atoms. The polymer may be syndiotactic orisotactic, however it is preferable to use an isotactic polypropylenehomopolymer having an isotactic index of greater than 0.85, morepreferably greater than 0.92, the articles obtained from saidhomopolymer having better physical properties. The melt flow index ofthe polymer will be from about 0.3 to about 10, preferably from about1.0 to about 4.0 g/10 min., determined according to ASTM D1238,Condition L. Such polymers are readily prepared by a variety ofcatalyzed polymerization processes well known in the art, includingprocesses employing Ziegler-Natta catalysts and those based onmetallocene catalysts.

A wide variety of extrusion grade, film-forming, substantially linearpolypropylene resins without significant branching having an essentiallymonomodal molecular weight distribution and the requisite MFR arereadily available in the trade and most will be found useful forproviding foam sheet having improved surface appearance according to theteachings of this invention.

According to the teachings of the prior art, it has been generallybelieved that high melt strength resin formulations are required inorder to successfully extrude foamed polypropylene having good cellstructure and acceptable surface appearance, and the art has developedspecialty formulations comprising particular grades of propylene resinshaving particularly defined molecular weight and rheological propertiesincluding a bimodal molecular weight distribution comprising a highlybranched minor component for these uses. Blend compositions having therequisite melt strength have also been formulated using polypropylenethat has been modified, for example through crosslinking, or withparticular polymeric additives, highly branched olefin polymers or thelike. Although these specialty resin grades and resin formulations mayalso be found suitable for use in the practice of this invention, foamsheet having improved surface appearance may be readily produced fromreadily available generic grades of polypropylene, i.e. propylenepolymer resins with monomodal molecular weight distributions and withouta significant level of branching, hence such specialty resincompositions are not required.

As disclosed and described in the art, foamable polypropylenecompositions will further comprise a blowing agent and a crystallizationnucleating agent.

The blowing agent may be of the type well known and widely used for theproduction of expanded polystyrene and polyolefins includingpolypropylene, including organic blowing agents such as, for example,azodicarbonamide, diazoaminobenzene, azo-bis-isobutyronitrile andanalogs thereof, and inorganic blowing agents such as, for example,ammonium carbonate, sodium bicarbonate and the like. Physical blowingagents such as nitrogen, carbon dioxide and other inert gases and agentsthat undergo phase change from liquid to gas during the foaming processsuch as chlorofluorocarbons (CFC), HCFC, low boiling alcohols, ketonesand hydrocarbons, are also known for these uses and may also be founduseful in the practice of this invention. The blowing agent may furthercomprise one or more additives to reduce its decomposition temperature.

The amount of blowing agent to be used depends on its nature and on thedesired density for the expanded polypropylene and will be selectedaccording to practices well understood by those skilled in the resinformulating art. Generally, blowing agents are available to the trade inthe form of concentrates; the concentrates will be added to theformulation at levels that will provide from about 0.2 to about 10 wt. %active foaming agent, preferably from about 0.4 to about 5 wt. % activefoaming agent, based on total weight of the formulation. The amounts ofphysical blowing agents such as liquid blowing agents and inert gasesneeded to provide the desired foam densities may readily be determinedaccording to common commercial practice.

As set forth in the art, a crystallization nucleating agent is providedto increase the number of crystallization nuclei in the moltenpolypropylene, thereby increasing the crystallization speed andpromoting crystallization from the melt, solidifying the resin at ahigher temperature. Generally, non-nucleated polypropylene will begincrystallizing at around 120° C. with a peak in crystallization rate near110° C. Nucleated polypropylene resins may start to crystallize attemperatures as great as about 135 to 140° C., with a peak around 130°C. Nucleated resin will solidify rapidly with improved melt strength tothereby reduce sag in the extruded foam sheet. The crystallizationnucleating agent will generally be used in an amount of from about 0.01to about 0.5 wt. %, preferably from about 0.05 to about 0.3 wt. %.Examples of such agents disclosed in the art and employed for improvingthe crystallization speed include organic sodium phosphates such assodium bis(4-tert-butyl-phenol)phosphate, sodium benzoate and mixturescomprising a monocarboxylic aromatic acid or a polycarboxylic aliphaticacid and a silicate or an alumino-silicate of an alkali or alkalineearth metal. The use of organic sodium phosphates as crystallizationagents is disclosed in the art, for example in U.S. Pat. No. 4,596,833.

Other agents disclosed in the art for improving melt strength ofpolyolefins include sorbitol, dibenzilidene sorbitol and relatedcompounds. These agents have been described in the art as networkingagents for use in modifying the low shear melt viscosity and low shearmelt strength of polyolefins and as crystallization nucleating agents.However, as will be seen, such networking agents are ineffective inproviding high density, rigid, foamed polypropylene sheet according tothe invention.

Further modifiers and additives for foamable polypropylene compositionsare also disclosed and described in the art. Such modifiers andadditives may be employed in amounts according to the common practice inthe art, including lubricants, coloring and/or drying agents,fire-proofing agents, thermal and UV stabilizers, antioxidants,antistatic agents and the like. It will be understood by those skilledin the art that such additional modifiers and additives will be selectedto avoid undesirable interaction with the resin, blowing agents andnucleating agents, and will be used at levels appropriate to theirfunction and purpose according to common practice in the foam resincompounding and formulating arts.

The improved foamable polypropylene compositions of this invention willfurther comprise a bubble nucleating agent. Bubble nucleating agentscreate sites for bubble initiation and desirably influence cell size andminimize the occurrence of large bubbles and open-cell structure,thereby providing particularly attractive and uniform high quality foamsheet. Use of a bubble nucleating agent in combination with acrystallization nucleating agent also further improves foamprocessability and melt rheology, as well as desirably enhancingimportant mechanical and thermal properties of the foam sheet,particularly rigidity or stiffness.

The bubble nucleating agent employed in formulating improvedcompositions useful for foam sheet extrusion according to the inventionmay be selected from the variety of inert solids disclosed in the art tobe useful as bubble nucleating agents, including mixtures of citric acidand sodium bicarbonate or other alkali metal bicarbonate, talc, silicondioxide, diatomaceous earth, kaolin, polycarboxylic acids and theirsalts, and titanium dioxide. Other inert solids disclosed in the art forthese purposes may also be found suitable. The nucleating agent willpreferably have a mean particle size in the range of from about 0.3 toabout 5.0 microns (μm), and will be present at a concentration of up toabout 5 wt. %, preferably from about 0.01 to about 5 wt. %, and morepreferably from about 0.5 to about 2 wt. % of the total weight of theformulation. At higher concentrations the cell structure becomesundesirably small; further, the nucleating agent tends to agglomerateduring processing.

A variety of compounding and blending methods are well-known andcommonly used in the art and most may be adapted to mix and compound thecomponents of foamable polypropylene formulations. Conveniently, theresin together with stabilizers and further additives and modifyingcomponents that are not thermally sensitive, whether in powder, pellet,or other suitable form, may be mixed and melt compounded using a highshear mixer, e.g., a twin-screw extruder at temperatures effective torender the resinous components molten and obtain a desirably uniformblend. Thermally sensitive components of the formulations, includingblowing agents, may be physically mixed with the resin in powder orpellet form using conventional dry-blending methods just prior tofeeding the mixture to the extruder. Plasticating the resin in acompounding extruder and feeding the additives and modifying componentsto the molten composition through a port in the extruder is alsocommonly practiced in the art. Downstream addition to the melt also maybe found particularly useful for foam sheet extrusion where a physicalblowing agent in the form of a gas is employed.

Processes for extruding foam sheet generally employ an extrusionapparatus having single or multiple extruders, which may be single ortwin screw extruders, to conduct the mixture of polypropylene resin andadditives through the plasticating and mixing steps, and provide amolten, foaming or foamable resin mass to the inlet of a sheet extrusiondie. Preferably the extrusion die will be a coathanger type sheetextrusion die wherein the inlet extends to a coathanger-shaped plenum inthe form of a relatively wide and vertically narrow cavity, elongated inthe horizontal or transverse direction (Y axis) and relatively narrow inthe vertical direction (Z axis). The resin flow direction or machinedirection may also be referred to as the extrusion axis (X axis). Theplenum is in liquid communication with an exit port or mouth extendingalong the width or Y axis of the die, forming a slit defined by dielips. Molten, foaming resin enters the die through the inlet, is spreadacross the width of the die by way of the plenum, passes between dielips and exits through the exit port or die exit in a molten orsemi-molten state as a continuous sheet. The extruded sheet will then becooled to become solidified, for example by being passed through a rollstack to cool the foam and finish the sheet. Differential roll speedsand take-up speeds may be employed to draw the foam sheet, orienting thecrystalline polypropylene and achieving a final form and thickness forthe sheet.

Turning now to the drawings, it may be seen in FIG. 1 that a typicalcoathanger die, generally designated by the reference numeral 10, forextruding thermoplastic sheet will comprise a first or lower half 1 anda second or upper half 2, indicated in phantom. Assembled in opposedrelationship, the halves form cavity 3. Molten foaming resin enters thedie through inlet 4 and flows into coathanger-shaped plenum 5. Plenum 5spreads the molten resin uniformly across the width of the die as itflows through the preland passage 6 to melt well 7. Adjustable chokemeans (not shown) may be included to provide control of resin flow, andany differences in pressure still remaining may be evened out by meltwell 7. The molten resin continues through planar extrusion passagewayor die land 9 defined by the opposing, spaced apart faces of lower dielip 11 and upper die lip 12, exiting the die through exit opening 8 andforming a sheet.

It will be understood that the die body may include passages for heatingand cooling, and further that clamping and fastening means and means forassembling the die to the extruder, also required, have been omittedfrom the drawings for clarity.

In FIG. 2 it will be seen that Prior Art sheet extrusion die 20 includesassembled upper and lower halves 21 and 22, together defining cavity 23,and upper and lower die lips 14 and 15. As described above, moltenfoaming resin will be supplied under pressure by extruder means (notshown) to die cavity 23 through inlet 24, in fluid communication withexit opening 28. Flowing into the coathanger plenum 25, and dammed bythe narrowing of the cavity at preland passage 26, the melt stream isspread across the width of the die by plenum 25 and fills melt well 27.The molten resin, further regulated by adjustable choke means 61, flowsfrom melt well 27 and passes through extrusion passageway or die land29, exiting the die through exit opening 28 as continuous foam sheet.

Either or both of die lips 14 and 15 may be made fixed or removable asdesired. As shown in FIG. 2, lower die lip 14 is made removable, securedto the lower body portion 12 by a plurality of bolts or other suitablefastening means, while upper die lip 15 is fixed. Upper die lip 15 maybe provided with adjusting means 62 as shown, for the purpose ofadjusting the gap between the die lips at exit opening 28.

In prior art sheet extrusion dies such as shown in FIG. 2, the opposingfaces of the upper and lower die lips 14 and 15 that make contact withthe molten resin stream are planar and substantially parallel, defininga smooth extrusion passageway or die land 29 having a substantiallyuniform height or thickness. The continuous foam sheet exiting the dielips in a molten or semi-molten state will have sufficient go internalpressure to undergo further expansion on exiting, reaching a finalthickness and surface condition when the temperature of at least theouter skin falls below the crystallization temperature and solidifies.

To complete the finishing of the extruded foam sheet, conventional sheetextrusion processes generally employ a finishing roll stack (not shown)which may contain chilled rolls to further cool the sheet. Optionally,cooling means such as an air stream-may be provided at the exit port toquickly cool the surfaces of the emerging foam sheet. Though initiallyshaped by the exit opening 28, the final thickness of the foam sheetwill thus depend in part on the cooling means employed, the roll gapwithin the finishing roll stack, and the ratio of the extrusion rate tothe takeup rate, which may be selected to draw the sheet as desired andorient the crystalline polypropylene component of the foam sheet.

In FIG. 3, showing an embodiment of an improved sheet extrusion dieaccording to the invention, the improvement over the prior art die ofFIG. 2 will be seen to reside in the modification of die lip 16 which,together with upper die lip 17, will provide a die land 39 with wideningaperture downstream along the extrusion axis from near melt well 37 toexit opening 38, thereby providing expansion zone 65. Molten foamingresin flowing from melt well 37 of die 30 enters expansion zone 65,expands under the influence of the pressure drop and exits the exitopening or die port as a continuous foam sheet. Foam expansion to agiven thickness is substantially completed within the die land. Byavoiding any significant further expansion after exiting the die port,surface banding, corrugation and similar surface markings will besignificantly reduced and may be entirely eliminated. Such defects arecommonly seen in foam sheet extruded using prior art extrusion dieswithout such an expansion zone such as, for example, the die shown inFIG. 2.

It may be found desirable to aid the cooling of the foam sheet while inthe expansion zone 65, thereby speeding crystallization, and provisionmay be made for including cooling means in the upper and lower die lips16 and 17 near exit opening 38 for this purpose. Finishing rolls, drawrolls and the like may be employed as desired to complete the processand finish the foam sheet to a final thickness and form.

In FIG. 4, showing an alternative embodiment according to the invention,die land 49 is provided with a widening aperture downstream along theextrusion axis by modification of lower die lip 18 and upper die lip 19,thus providing an expansion zone 65 whereby foam expansion to a giventhickness may be substantially completed within the die land.

The geometry of extrusion dies will be designed for particularconditions characterized from the rheology of the melt. It will bereadily understood that placing particular numerical values on thevariation of die gap geometry in the manner set forth above would beunduly limiting in that the die opening or die gap is a function of thedesired final foam product thickness as well as many other factors. Ifone skilled in the art were to determine the proper die geometry andsize, and the process throughput rate to produce a given product, theappearance and uniformity of the sheet product are subject toimprovement as described as a function of the modifications of the dielip according to the invention.

In defining a complete, improved process of producing a foamedpolypropylene sheet according to the invention, one must take intoaccount the die gap or opening and the length of the die land orextrusion passage, as well as operating parameters including throughputrate, product size and bulk density. The geometry parameters are notdetermined independent of the operating parameters.

Turning to FIG. 5, wherein the die land of FIG. 4 is shown in enlarged,fragmentary, view, it will be seen that the widening aperture of dieland 49 may be further characterized by the height or thickness t₁ ofthe passage at die exit opening 48 and the height or thickness t₂ at thenarrow point, i.e. the minimum height within the die land. Generally,the initial thickness of the extruded foam sheet will be determinedsubstantially by the height of the exit opening 48. Hence, thickness t₁may be as small as 0.01 inch (0.025 cm) to as great as 0.2 inch (0.5 cm)or greater, depending upon whether the die is intended for theproduction of thin, dense, expanded film-like sheet or thicker foamsheet, and may be even greater where the die will be used in producingfoam board or the like. To be suitable for the production of foamedsheet having acceptable surface appearance according to the invention,the geometry may be characterized in part by the difference (t₁−t₂),i.e. the difference between the height or thickness t₁ of the passage atexit opening 48 and the height or thickness t₂ at the minimum heightwithin the die land, which will lie in the range of from about 0.004 toabout 0.10 inch (0.01-0.25 cm). Alternatively described, the geometry ofthe die land for producing suitable foam sheet over the range ofinterest may be characterized by the ratio t₁/t₂ between the height orthickness t₁ of the passage at exit opening 48 and the minimum heightwithin the die land t₂, which will be in the range of from about 2 toabout 5.

The geometry of the die land suitable for producing foam sheet accordingto the invention may be further characterized by length l₁ of die landor passageway 49, determined from the downstream edge of melt well 47 tosaid exit opening, and length l₂ of the widened portion formingexpansion zone 65. Generally, the length of the initial, narrow portionof the die land, defined by the difference (l₁−l₂), i.e. by thedifference between the length of die land 49 and the length of theexpansion zone 65, will lie in the range of from about 0.125 Inch toabout 0.75 inch (0.3-1.9 cm).

These parameters will be found to serve generally for the design ofimproved extrusion dies to meet the requirements for producing extrudedfoam sheet according to the invention over a wide range of productionvolumes, including the embodiment shown in FIG. 3.

In the operation of the foam sheet extrusion apparatus, upper die lipadjustment means may be employed to modify the die gap or opening withina narrow range to further control sheet thickness. The improved sheetextrusion die according to the embodiment of FIG. 4 may be operatedusing a take-up rate selected to remove the foam sheet from the dieprior to expanding to the full height t₁ of the die exit opening 48,allowing the operator to adjust and maintain sheet thickness over arange limited by the minimum and maximum heights t₁ and t₂ within theaperture of expansion zone 65.

Foamed polypropylene sheet extruded employing the improved extrusion dieaccording to the invention will have improved surface appearance, withlittle or no surface banding, corrugation or similar surface markings,and may be further characterized as having a low level of surfaceroughness that is uniformly distributed, giving the foam sheet apleasing and acceptable cosmetic appearance.

Surface roughness and uniformity in surface roughness may be determinedby image analysis and thus quantified, thereby providing a numericalbasis for distinguishing acceptable from unacceptable foam sheet. Moreparticularly, rough surfaces reflect light nonuniformly; areas of asurface that are smooth are more reflective, and in a gray scalephotomicrograph of a surface appear white or light gray. Areascontaining defects such as the voids and indentations that formcorrugation bands will scatter light and thus appear in a gray scalephotomicrograph as dark gray or black. When digitized using computerizedimage analysis methods and translated to a binary, black/white image,the average surface roughness of the foam sheet will be related to theamount of black in the binary image, determined as a fraction of thetotal surface area.

The uniformity of the distribution of roughness over the surface may bealso determined from the photomicrograph, again by image analysis. Theuniformity of surface roughness will be related to the variation inroughness over the area of the photomicrograph, determined by examiningequal areas of the image, averaging the roughness for each area, andthen obtaining the standard deviation. The surface having greatestuniformity will be the surface having the lowest standard deviation.

Foam sheet with a low average surface roughness, uniformly distributedover the surface as reflected by a low standard deviation in roughnessover the examined area generally will be considered to have anacceptable appearance.

It will be understood that improved sheet extrusion dies according tothe invention may also be found useful for the extrusion of unfoamedpolypropylene sheet, or of multilayered foam sheet or foam core sheet,and further that foam extrusion dies having a die land including anexpansion zone according to the teachings hereof may be made inalternative configurations, including improved annular dies and inconfigurations suitable for the extrusion of foam board and plank, andin profile extrusion of shaped foam structures with improved surfaceappearance.

The methods and processes of this invention, and the formulations andimproved extrusion die employed therein, may be used for the productionof improved foamed polypropylene over a wide range of densitiesincluding low density, flexible foam for packaging uses, and for themanufacture of foamed polypropylene plank and board with thicknessesgreater than ¼ inch (0.6 cm) to as great as 1 inch (2.5 cm) or more.Rigid, foamed polypropylene sheet having a thickness in the range offrom about 10 mils (0.25 mm) to about 250 mils (6 mm), preferably fromabout 20 mils (0.5 mm) to about 80 mils (2 mm), and with foam densitiesin the range of from about 0.4 g/cm³ to about 0.8 g/cm³, preferably fromabout 0.45 g/cm³ to about 0.8 g/cm³ and still more preferably from about0.5 to about 0.75 g/cm³, will be particularly preferred. The rigidity ofthe high density foam sheet of this invention is reflected in the hightensile modulus of the foam sheet, generally above about 150,000 psi(1000 MPa), more particularly in the range of from about 150,000 psi toabout 300,000 psi (1000-2000 MPa).

At lower densities, foam polypropylene sheet is more flexible and lacksthe rigidity desired for rigid food packaging uses and the like. Thus,the lower density polypropylene foam sheet disclosed in U.S. Pat. No.5,149,579 is characterized by patentees as having tensile and flexuralmoduli values in the range of from 10,000 to 50,000 psi (70-340 MPa). Atfoam densities above about 0.8 g/cm³, foam polypropylene sheet will lackthe thermal insulation characteristics desired for many applications.

Thin, rigid foam sheet produced according to the invention will alsohave good thermal insulating properties, with a thermal conductivity inthe range of from about 0.08 W/m° K. to about 0.15 W/m° K. Foam sheethaving a high degree of rigidity in combination with good thermalinsulating properties will be particularly attractive and important foracceptability in food service applications, for example, for use in themanufacture of cups and similar containers for handling hot or chilledliquids, and where food must be maintained at temperatures significantlyabove or below ambient temperature for some period of time. Foamintended for these uses will preferably have a thermal conductivitybelow about 0.14 W/m° K. and more preferably below about 0.11 W/m° K.

Polypropylene foam sheet produced from the improved polypropylenecompositions according to the invention may be used in a conventionalthermoforming operation to form rigid and semi-rigid articles.Typically, articles are formed from sheet having a thickness of fromabout 10 mils (0.25 mm) up to 200 mils (5 mm) or above. A thermoformedarticle typically ranges from about 20 to 80 mils (0.5-2 mm). Generally,processes for thermoforming foam sheet include the steps of heating thefoam sheet to a temperature where it is deformable under pressure orvacuum, supplying the softened foam sheet to a forming mold, and coolingthe foam sheet to form a rigid or semi-rigid article having the shape ofthe mold. To avoid collapsing the foam structure of the sheet, thetemperature employed in the heating step will fall in a narrow rangewhich does not exceed the melt temperature of the resin. The processingwindow or temperature range for thermoforming, and particularly theupper temperature limit, may be conveniently assessed by athermomechanical analysis procedure whereby a sample of the sheet isheated while monitoring the change in thickness of the sheet as afunction of temperature, using a thermomechanical analyzer probe. Uponreaching and then exceeding the upper limit of the processing range, thethickness of the sheet will be observed to rapidly decrease as the foamstructure collapses and the probe penetrates the sheet. Generally,extruded foam sheet comprising polypropylene may be processed with goodretention of foam structure at temperatures of from about 130° to about145° C., and particular formulations may be found to be processable attemperatures as great as 150° C. while retaining foam structure.

In another aspect of the invention, a surface layer may be applied byco-extrusion techniques. Typically, top and bottom surface layers withthickness ratios of the layer to the foam core of about 1:1000 andpreferably 1:2000 or above may be used. Preferably, the surface layer isa propylene polymer with a similar composition (except for blowingagents) to the foam core, although any compatible propylene polymer maybe used. Also, if desired a barrier resin layer also may be applied suchas polyethylene or ethylenevinylacetate polymer. An advantage of usingco-extruded surface layers is incorporating pigments or otherspecialized additives to the surface layers. Since the amount of surfacelayer is much smaller than the foam core, the use of pigments or otheradditives is minimized. This may be beneficial in recycling the article.

Extruded polypropylene foam sheet having improved surface appearanceaccording to the invention has application in a wide variety of physicalshapes and forms in addition to molded goods. Rigid and semi-rigidfoams, including molded and laminated products prepared therefrom, notonly possess good physical properties and excellent chemical resistanceat room temperature, but they retain their strength and good performanceover a wide range of temperatures and for long periods of time. Moldedarticles formed from the preferred foam composition of this inventionhave markedly improved surface appearance and may be particularly usefulin food packaging where appearance and cosmetic considerations arehighly important to consumer acceptance. Examples include plates, cups,trays, and containers such as for take-out food and home mealreplacement items. Since these articles are made from propylene polymerwith a relatively high softening point, the articles typically may beused in a microwave oven. The foam sheet and molded articles may alsofind wide Use in applications where mechanical strength, rigidity andthermal insulation are important considerations, such as in durablegoods and appliance components, and in medical and plumbing applicationswhere resistance to hot, humid environments may be particularlyimportant, as well as in safety equipment and protective gear.

The invention will be better understood by way of consideration of thefollowing illustrative examples and comparison examples, which areprovided by way of illustration and not in limitation thereof. In theexamples, all parts and percentages are by weight unless otherwisespecified.

EXAMPLES

The PP resins employed in the following examples were prepared using theAmoco Gas Phase Process. The process is disclosed generally in“Polypropylene Handbook” pp. 297-298, Hanser Publications, NY, 1996, andis more fully described in U.S. Pat. No. 3,957,448, the teachings ofwhich are incorporated herein in their entirety by reference thereto,and in “Simplified Gas-Phase Polypropylene Process Technology” presentedin Petrochemical Review March, 1993.

PP resins are initially produced in powder form. The resin powder may beused directly, or may be first compounded and pelletized by strandextrusion using a compounding extruder, and then chopping the strand.Pelletizing may be accomplished according to standard practice, forexample by dry-blending dried resin with such stabilizing components andadditives as may be required and feeding the blend to a ZSK-30twin-screw extruder. The polymer, extruded through a strand die intowater, is then chopped to form pellets. PP resins are generallycharacterized by molecular weight; among the measurements employed fordescribing resin molecular weight, in addition to resin viscosity, isthe resin Melt Flow Rate or MFR. Generally, molecular weight isinversely related to MFR.

The component materials employed in following examples, and theabbreviations therefor, include:

Polypropylene Resins

PP-1: propylene homopolymer, powder, MFR=2 g/10 min.

PP-2: propylene homopolymer, powder, MFR=1 g/10 min.

PP-3: propylene homopolymer, pelletized, MFR=2 g/10 min.

PP-4: propylene-ethylene copolymer, pelletized, MFR=2 g/10 min.

Crystal Nucleating Agents

Nucl-1: sodium benzoate

Nucl-2: organic sodium phosphate, obtained as MARK NA-11 from AdekaArgus Chemical Company

Nucl-3: dimethyldibenzylidene sorbitol, obtained as Millad 3988 fromMilliken Chemical Company

Blowing Agents

FPE-50: Proprietary sodium bicarbonate-based blowing agent, obtained asSAFOAM FPE-50 from Reedy International Corporation, added asconcentrate, 50 wt. % active.

CF-40E: Proprietary sodium bicarbonate-based blowing agent, obtained asBI CF-40E from BI Chemical Company, added as concentrate, 40 wt. %active.

H-40E: Proprietary sodium bicarbonate-based blowing agent, obtained asBI H-40E from BI Chemical Company, added as concentrate, 40 wt. %active.

Bubble Nucleating Agent

Talc: talc having a 0.8 micron mean particle size, maximum particle size6 microns, obtained from Specialty Minerals Inc. as Microtuff AG-609 andAGD-609 grades.

Test methods employed in evaluating the foam sheet produced in thefollowing Examples include:

Melt Flow Rate (MFR) was determined by ASTM D1238, Condition L (230° C.,2.16 Kg load).

Density was determined according to ASTM D1622.

The crystallization temperature (Tc) and melting temperature (Tm) weredetermined by differential scanning calorimetry following substantiallythe procedures of ASTM E-793.

Thermal conductivity was determined using a C-matics model TCEM-DVinstrument from Dynatech Corporation following test procedures publishedby the manufacturer.

Tensile testing was carried out substantially according to theprocedures of ASTM D638.

Evaluation of cell dispersion and size was made by scanning electronmicroscopy (SEM) on cross sections microtomed from PP foam sheet.Specimens were taken along the transverse and machine directions. Thecut specimens were mounted on SEM stubs, coated with Au/Pd, and examinedon a Hitachi S-4000 scanning electron microscope in secondary electronimaging mode, accelerated voltage 10 kV. Cell distribution wassubjectively rated visually on a basis of excellent, very good, good,fair, poor and very poor uniformity.

Examples 1-6

In Examples 1-6 and Control Examples C-1-C-3, powdered propylenehomopolymer resin having an MFR of 1.0 (PP-1), was compounded andextruded into foam sheet substantially by the following generalprocedure. The formulations are summarized in the following Table I,wherein all components are in wt. % based on total weight of theformation; balance of the formulation is PP-1. Examples 1-6 will be seento comprise a bubble nucleating agent (talc) and a crystallizationnucleating agent; the formulations of Control Examples C-1-C-3 containonly a bubble nucleating agent, without a crystallization nucleatingagent.

Polypropylene powder and the indicated amount of additives other thanblowing agent were dry blended in the amounts indicated, fed to a ZSK-30compounding extruder, melt mixed and extruded to form pellets. The resinpellets were dry-blended with the indicated amount of blowing agent andgravity fed through the hopper to a 2.5 inch (6.4 cm) single-screw NRMextruder, having a 24/1 UD screw, with barrel and die heating. Thebarrel of the extruder was maintained at a temperature in the range ofabout 360° to about 380° F. (182-193° C.). The molten, foaming mixturewas extruded through a 12 inch (30.5 cm) width coathanger die maintainedat 380° F., pulled in an S-wrap through a three-roll finishing rollstack maintained at a temperature of approximately 130° F. (54° C.),located approximately 6 inches (15 cm) from the die exit, and taken upon a winder. For the purposes of these examples, the take-up speedemployed was selected to provide only nominal drawing and orientation ofthe polypropylene foam in the machine or flow direction.

An improved extrusion die substantially as shown in FIG. 3 and having aratio t₁/t₂=1.2 and a ratio l₁/l₂=3.5, was employed for extruding thefoam sheet of Examples 1-6 and C-1-C-3. Foam sheet for each of theformulations was extruded under low shear conditions and also under highshear conditions to demonstrate the effect of mixing on celldistribution and uniformity. A low extruder screw speed of 86 rpm wasemployed for low shear mixing, and a higher extruder speed of 120 rpmfor high shear mixing.

Samples of the foam sheet were taken for determination of mechanical andthermal properties as described below.

All formulations set forth in the following Table I also contain 0.08wt. % Irganox 168 and 0.04 wt. % Irganox 1010 stabilizers, from CibaGeigy. Examples 2-4 contain 0.02 wt. % Kyowa DHT4A grade of synthetichydrotalcite, obtained from Mitsui, to neutralize acidic catalystresidues. The resin component of these formulations (balance to 100 wt.%) is PP-1 propylene homopolymer. All formulations were combined with0.5 pbw (active) SAFOAM FPE-50 blowing agent per hundred partsformulation before extruding.

TABLE I Example 1 2 3 4 5 6 C-1 C-2 C-3 Talc (%) 0.6 0.75 0.9 1.1 0.750.75 0.6 0.75 0.9 Nucl-1 (%) 0.1 0.1 0.1 0.1 — — — — — Nucl-2 (%) — — —— 0.06 — — — — Nucl-3 (%) — — — — — 0.17 — — — MFR (gl/10 min.) 2.632.97 3.47 3.11 3.09 3.10 2.93 2.79 2.95 Tc (° C.) 122 124 124 125 128121 121 121 121 Tm (° C.) 160 160 160 160 160 161 161 160 160 Notes: Forcompounding details, test methods, see text.

The presence, as in Examples 1-6, or absence, as in Control ExamplesC-1-C-3, of a crystallization nucleating agent is without effect on themelting temperature Tm. Crystallization temperature Tc for theuncompounded polypropylene employed for these Examples, PP-1, 127° C.;however, as will be seen in the Control Examples C-1-C-3, Tc is reducedto about 121° C. by the addition of talc. Thus, nucleating agents Nucl-1and Nucl-2 serve to overcome the effect of talc on crystallizationtemperature Tc, and Nucl-2 provides a further improvement in Tc. CompareTc for Examples 1-5 with C-1-C4.

Sorbitol compounds, described in the art as networking agents suitablefor nucleating crystallization in polyolefins, appear to provide littleimprovement in crystal melt temperature for foamable formulationsaccording to the invention based on polypropylene.

Density and thermal conductivity determined for samples of foam sheet,extruded at low and at high shear conditions for each of theformulations as described above, are summarized in Table II, below.

TABLE II Low Shear High Shear Thermal Thermal Ex. Density cond. Densitycond. No. Kg/m² W/m ° K. Cells Kg/m² W/m ° K. Cells 1 0.71 0.15 v. good0.63 0.11 v. good 2 0.90 0.19 poor 0.83 0.14 poor 3 0.79 0.17 fair 0.670.12 good 4 0.83 0.15 fair 0.72 0.13 fair 5 0.75 0.15 good 0.78 0.14good 6 0.78 0.13 fair 0.74 0.13 fair C-1 0.71 0.12 fair 0.74 0.12 fairC-2 0.75 0.13 fair 0.67 0.12 fair C-3 0.75 0.12 good 0.71 0.11 goodNotes: For compositions, test methods, see text and Table 1.

The effect of high shear mixing in the extruder is to increase melttemperature, thus improving cell distribution and reducing average cellsize. It will be apparent from comparing the densities of foams producedin low shear conditions with the corresponding foams produced under highshear conditions that high shear mixing conditions generally provide alower density foam with more uniform cell distribution as reflected inbetter insulating properties (reduced thermal conductivity).

The mechanical properties for low shear and high shear foam sheet ofExamples 1-6 and Comparison Examples C-1-C-3 were also determined. Thetensile properties are summarized in the following Table III. Sampleswere tested in the machine direction.

TABLE III Low Shear High Shear Tensile Tensile Tensile Tensile Yield EMod. Yield E Mod. Ex. Kpsi Yield Kpsi Kpsi Yield Kpsi No. (MPa) % (MPa)(MPa) % (MPa) 1 2.83 4.3  212 2.24 5.3  161 (19.5) (1460) (15.4) (1110)2 4.52 5.1  289 4.09 5.9  274 (31.2) (1990) (28.2) (1890) 3 3.95 4.5 266 2.78 5.5  198 (27.2) (1830) (19.2) (1370) 4 3.60 4.2  253 2.44 5.2 181 (24.8) (1740) (16.8) (1250) 5 3.42 4.4  241 3.58 5.5  236 (23.6(1660) (24.7) (1630) 6 3.21 4.2  246 3.58 7.9  194 (22.1) (1700) (24.7)(1340) C-1 3.03 5.5  208 3.33 5.4  227 (20.9) (1430) (23.0) (1570) C-22.98 4.9  195 2.98 5.9  198 (20.5) (1340) (20.5) (1370) C-3 3.33 5.0 217 2.98 6.3  189 (23.0) (1500) (20.5) (1300) Notes: For compositions,test methods, see text and Table 1.

Examples 7-12

In the following Examples 7-12, the formulations summarized in Table IVwere compounded and extruded substantially as described for Examples1-6, using a screw speed of 120 rpm to assure good mixing. An improvedextrusion die, substantially as shown in FIG. 3 and having a ratiot₁/t₂=1.2 and a ratio l₁/l₂=3.5, was employed for extruding the foamsheet.

The formulations also contain 0.08 wt. % Irganox 168 and 0.04 wt. %Irganox 1010 stabilizers, from Ciba Geigy. The balance of theformulation (to 100 wt. %) is the indicated resin component. Theformulations were combined with 0.5 pbw (active) blowing agent perhundred parts formulation before extruding.

TABLE IV Example 7 8 9 10 11 12 PP PP-2 PP-1 PP-1 PP-3 PP-2* PP-2* Talcwt.% 1 — 1 1 0.75 0.75 Nucl-1 wt % 0.1 0.1 0.1 0.1 — — Nucl-2 wt % — — —— 0.08 0.08 Blowing FPE-50 FPE-50 FPE-50 FPE-50 CF40E H-40E AgentThickness mil 35 40 42 53 35 38 mm 0.89 1.02 1.07 1.35 0.89 0.97 Cellsize, 90 120 120 160 130 210 ave. μm Cell dispersion good v. good fairgood v. good poor SEM Surface — — — no no voids, appearance voids,voids, v. good excl. poor Thermal cond. 0.11 0.12 0.12 0.13 0.11 0.11W/m ° K. Density g/cm³ 0.65 0.65 0.73 0.66 0.62 0.56 Notes: *Pelletizedresin. For compounding details and test methods, see text and Table I.

It will be seen that not all blowing agents provide equivalent results,the H-40E blowing agent giving foam with poor cell dispersion and large,irregular, open and interconnected cells when viewed by SEM (Example12). Although the surfaces of the foam sheet of Examples 7-12 had nosignificant banding, the foam sheet of Example 12 had a rough surface,with voids; apparently, these defects were the result of using a lesseffective, coarser particle foaming agent. The cells of Examples 7-11,viewed by SEM in cross section taken in the transverse direction,appeared to be essentially circular in cross section. Viewed in crosssection along the machine direction, the cells of these foams will beseen to be elongated along the direction of flow.

Foam sheet having thermal conductivity below about 14 W/m° K.,preferably below about 11 W/m° K., are particularly desirable for use infood service applications, particularly in the production of cups andsimilar articles for use in storing and serving hot or cold foods. Forcomparison, the thermal conductivity of extruded unfoamed, stabilizedPP-2 polypropylene sheet having a nominal thickness of 40 mils (1 mm) is0.20 W/m° K. The thermal conductivity of stock used for producing papercups is about 0.12 W/m° K., while for Styrofoam cup stock the value isabout 0.09 W/m° K.

Comparison Example C4

For comparison purposes, foam sheet was extruded using a prior art dieand the foamable polypropylene formulation of Example 11, set forth inTable IV. The prior art extrusion die was substantially as shown in FIG.2. The foam sheet had an average thickness of 35 mils (0.9 mm) and anaverage density of 0.62 g/cm³.

The foam sheet of the Comparison Example C-4 was seen to have poor celldispersion when viewed by SEM and, on visual inspection, the surface ofthe foam sheet was visibly flawed, with bands running in the machinedirection, defects typically seen in foam sheet extruded fromcommercial, unmodified polypropylene resins using prior art dies.

As noted in Table IV above, when inspected by SEM, the surfaceappearance of the invented foam sheet of Example 11 was seen to beexcellent, with no observable surface voids. On visual inspection, thefoam sheet of Example 11 had no visible banding or other significantsurface defects.

Surface appearance of various foam sheet specimens may be furthercompared by rating the surface roughness and uniformity for each usingroughness parameters obtained through image analysis of photomicrographsof the surface of the sheet. Specimens of sheet suitable for rating areextruded substantially as in Examples 10 and 11; for comparisonpurposes, representative specimens of unacceptable foam sheet areprovided as described in the Comparison Example C-4 and rated.Representative specimens of foam sheet, summarized in Table V asExamples A-C, are produced to provide further comparisons by extrudinggeneric polypropylene resins using prior art processes as in theComparative Example C-4.

In making the ratings, an area of the sheet surface approximately 5mm×16 mm was viewed by SEM and photographed. The examined area wasselected to include obvious surface roughness, together with acorrugation, line if one existed. Visual appearance ratings were thendetermined by image. analysis of 16 equal areas, each 1 mm wide by 5 mmin length, selected to run parallel to the corrugation line. Thefraction (%) of dark areas for binary images of each of the strips, thestandard deviation (%) in dark area for the 16 strips, and the averagedark area (%) for the total area examined, were determined. The resultsof these evaluations are summarized in a table V.

Average Roughness is related to the % of dark area for the total areaexamined; high values indicate the presence of more voids andindentations, i.e. a rougher appearing surface.

Variation in average roughness between areas of the surface indicatesnonuniform distribution of cells, voids and indentations on the surface.For a completely uniform surface, whether rough or smooth, there wouldbe little variation in average roughness between various areas of thesurface.

Standard deviation is related to the uniformity of the surfaceroughness, and indirectly to the uniformity of internal celldistribution; low values indicate more uniformly distributed cells,voids and indentations on the surface, and a more uniform distributionof cells internally.

The criteria for rating and indexing surface roughness and uniformityare summarized in tabular form as follows:

Roughness Uniformity Rating (Ave., %) (Std. Dev., %) 1 <10 3 2 10-20 3-43 20-30 4-5 4 30-40 5-6 5 40-50 6-7 6 50-70  7-10 7 >70 >10   

TABLE V Surface Roughness and Uniformity Ratings of RepresentativeSpecimens of Foamed Polypropylene Sheet Specimen according to Ex.: 10 11C-4 A B C Roughness, Ave. % 24.6 74.6 59.2 20.6 38.4 14.3 Roughnessrating: 3 7 6 3 4 2 Std. Deviation % 3.2 3.7 22.0 4.4 5.2 7.0 Uniformityrating: 2 2 7 3 4 6 min. % 18.4 68.0 29.3 14.1 26.2 5.8 max. % 32.6 79.394.3 30.2 47.5 30.7 Visual Appearance: B C E C D F Notes: % values are %of area; see text. Visual Appearance; A excellent; B good; C very lightcorrugation; D corrugation lines fine, clear; E corrugation lines thick,clear; F very non-uniform with no corrugation

It will be seen that the primary characteristic of foam sheet having anacceptable surface appearance is uniformity in the distribution ofsurface roughness. When the surface roughness is uniformly distributed,whether low as in Example 10 or high as in Example 11, foamedpolypropylene sheet will have good visual appearance. Very faintcorrugation lines will result from a less uniform distribution ofsurface defects as in Example A. The corrugations become more noticableand have a deleterious effect on surface appearance as distributionbecomes more nonuniform. See Example B and the Comparison Example C-4.Conversely, low surface roughness, when distributed in a highlynon-uniform manner, provides sheet having a very undesirable surfaceappearance as in Example C.

Acceptable foam sheet will thus be seen to be sheet having a uniformdistribution of surface roughness, reflecting an overall uniformity ininternal cell distribution. For foam sheet according to the invention,the standard deviation in surface roughness, a measure of uniformity,will generally be less than about 6.0%, more preferably less than about5.0% and most preferably below about 4.0%.

The invention will thus be seen to be directed to an improvedpolypropylene composition for use in the production of extruded foamsheet, more particularly, rigid or semi-rigid foamed polypropylene sheethaving a thickness of from about 20 to about 80 mils(0.5-2 mm) and adensity of from about 0.4 to about 0.8 g/cm³. Rigid, foamedpolypropylene sheet according to the invention will be significantlyimproved in surface appearance, with the substantial absence of thebanding and corrugation markings commonly seen in extruded rigidpolypropylene foam of the prior art. The invention may also becharacterized as directed to rigid foamed polypropylene sheet comprisinggeneric polypropylene as defined herein and having a density of fromabout 0.4 to about 0.8 g/ cm³, and a uniform distribution of surfaceroughness which may be further defined as a standard deviation in theaverage surface roughness generally below about 6%, more preferablybelow about 5.0% and most preferably below about 4.0%.

Although the invention has been described and illustrated by way ofspecific embodiments set forth herein, those skilled in the art willrecognize that a variety of polypropylene homopolymer and copolymerresins including impact-modified polypropylene resins such as, forexample, those disclosed in the art based on an isotactic polypropylenecontaining a dispersed phase comprising a copolymer may also be founduseful. Still further modifications and variations in the processesemployed herein for the production of foam sheet will be readilyapparent to those skilled in the resin formulating and fabricating artand in the extrusion arts, and such variations and modifications will beunderstood to lie within the scope of the invention as defined by theappended claims.

We claim:
 1. A rigid, extruded sheet having a density of from about 0.4to about 0.8 g/cm³ comprising foamed, substantially linear propylenepolymer having an essentially monomodal molecular weight distributionand a melt flow rate of from about 0.5 to about 30 g/10 min. determinedaccording to ASTM D1238 condition L, the surfaces of said sheet having astandard deviation in surface roughness of less than about 6%.
 2. Therigid, extruded sheet of claim 1 wherein said standard deviation insurface roughness is less than about 5%.
 3. The rigid, extruded, foamedsheet of claim 1 obtained by foam extrusion of a substantially linearpolypropylene resin having having an essentially monomodal molecularweight distribution and a melt flow rate of from about 0.5 to about 30g/10 min. determined according to ASTM D1238 condition L, said resinhaving dispersed therein from about 0.05 to about 0.5 wt. % of acrystallization nucleating agent, from about 0.01 to about 5 wt. % of afinely divided inert solid and from about 0.1 to about 25 wt. % of afoaming agent.
 4. The foamed polypropylene sheet of claim 3 having athickness of from about 10 mil to about 250 mil.
 5. The foamedpolypropylene sheet of claim 3 having a thickness of from about 10 milto about 80 mil.
 6. The foamed polypropylene sheet of claim 3 whereinsaid density is in the range of from about 0.45 to about 0.8 g/cm³. 7.The foamed polypropylene sheet of claim 3 herein said finely dividedinert solid has a mean particle size in the range of from about 0.3 toabout 5 microns and is selected from the group consisting of talc,silicon dioxde, diatomaceous earth, kaolin and titanium dioxide.
 8. Thefoamed polypropylene sheet of claim 3 wherein said crystallizationnucleating agent is selected from the group consisting of organic sodiumphosphates and sodium benzoate.
 9. The foamed polypropylene sheet ofclaim 3 wherein said crystallization nucleating agent is sodiumbenzoate.
 10. The foamed polypropylene sheet of claim 7 wherein saidinert solid is talc.
 11. A molded article comprising foamedsubstantially linear polypropylene resin, said article molded bythermoforming the extruded, rigid, foamed polypropylene sheet of claim3.
 12. An article of claim 11 wherein the thickness is about 20 to about80 mils.
 13. An article of claim 11 on which is co-extruded surfacelayers of a compatible polymer.
 14. An article of claim 11 on which isco-extruded surface layers of a propylene polymer which containspigment.
 15. An article of claim 11 in the form of a plate, cup, tray,or container.
 16. An article of claim 12 the surfaces of said articlehaving a standard deviation in surface roughness of less than about 5%.17. A molded article comprising foamed, substantially linear propylenepolymer having an essentially monomodal molecular weight distributionand a melt flow rate of from about 0.5 to about 30 g/10 min. determinedaccording to ASTM D1238 condition L, the surfaces of said molded articlehaving a standard deviation in surface roughness of less than about 6%.18. An article of claim 17 in the form of a plate, cup, tray, orcontainer.
 19. An article of claim 17 on which is co-extruded surfacelayers of a compatible polymer.
 20. An article of claim 17 on which isco-extruded surface layers of a propylene polymer which containspigment.